The present invention relates generally to food packaging and more particularly to paperboard based food packaging that includes a sealable, moisture barrier, product contact coating capable of producing fiber tearing seals when adhered together.
Packaging material of the type disclosed herein preferably comprises a paperboard substrate having a first side to which there has been applied a coating suitable for printing high quality graphics. Such substrates are known in the art as coated-one-side, or C1S paperboard substrates. It is also known in the art to employ for such packaging material a paperboard substrate to which there has been applied coatings suitable for printing high quality graphics on both sides. Such substrates are known in the art as coated-two-side, or C2S paperboard substrates. However, obviously, it is more expensive to utilize a C2S substrate than a C1S substrate for packaging, so there is a need in the art to develop C1S paperboard based packaging material for the intended purpose. Moreover, the use of a C2S substrate has been found to be less than desirable in providing fiber tearing seals when lids and trays prepared from such a substrate are heat sealed together, particularly with the use of some barrier materials.
In the case of paperboard based food packaging, at least one side of the paperboard substrate is preferably coated on the papermachine with a coating suitable for printing high quality graphics. Such coatings are usually applied in two steps. First, a base coat is applied, then a top coat is applied over the base coat. Compositionally, coatings useful for printing high quality graphics generally include a fluidized blend of minerals such as coating clay, calcium carbonate, and/or titanium dioxide with a suitable binder such as starch, polyvinyl alcohol, polystyrene or the like. These coatings are generally applied to the paperboard on a papermachine during the papermaking process by typical coating devices such as roll, rod, air knife or blade coaters. Successive densification and polishing of the coated surface by calendering finishes the surface to a high degree of smoothness and gloss to achieve a superior surface for high quality printing. In addition, at least one side of the paperboard substrate is also coated with a moisture barrier, product contact coating that is capable of being sealed either by heat sealing or with the use of microwave energy after the substrate is converted into food packages. Typical coatings used for this purpose in the past include extrudable resins such as low density polyethylene (LDPE), polypropylene (PP), and polyethylene terephthlate, (PET). However, the use of extrudable resins for such purpose must be done in a separate process, that is generally remote from the papermachine where most coatings are applied, which entails increased costs and handling. Moreover, the use of extruded resins for product contact coatings generally requires a thick film to achieve the desired moisture barrier protection and sealing results. Suitable and effective product contact coatings can also be applied to a paperboard substrate with the use of coatings prepared from emulsions such as acrylics, ethylene vinyl chloride (EVCL), polyvinylidene chloride (PVDC), and PET. However, the application of such coatings on a papermachine is generally not done because of the high costs associated with small orders and the high speed of papermachines.
In order to overcome these and other problems, the method disclosed in U.S. Pat. No. 5,418,008 was developed. According to the teachings of the '008 patent, a paperboard substrate which has been previously coated on one (C1S) or both (C2S) surfaces is applied with a continuous film of a moisture resistant, product contact barrier material, at a low coat weight, in a single pass, on a printing press, at the same time that the substrate is printed and converted into blanks for forming food packages. Unfortunately, the practice of the invention disclosed in the '008 patent has been discouraging due to poor coating holdout, drying problems and high coating consumption, particularly with the use of C1S paperboard, and with the inability to achieve consistent fiber tearing seals with the use of C2S paperboard. Accordingly, the present invention was developed to improve upon and overcome problems with the practice of the invention disclosed in the '008 patent.